In a communications network, request frames, such as a PS-Poll (Power Save Poll, which is a power saving mode frame), all have a defined frame structure, including a physical header part and a media access control layer (Media Access Control, MAC for short) part. Later, in further development, a new PS-Poll structure is proposed. This type of the new structure also performs a function of the PS-Poll, but is relatively simplified, reducing a load of network transmission. This type of the new structure only has a physical header part, and therefore, is called an NDP PS-Poll. NDP is the abbreviation of No Data Packet, that is to say, there is no data part, which means that there is no MAC layer part, and there is only a physical header.
After an NDP PS-Poll is used, because the NDP PS-Poll is relatively short, so that it is incapable of holding a 48-bit basic service set identifier (basic service set identifier, BSSID for short), namely a destination address, and may only use a short destination address, for example, 9-bit. The 9-bit address is calculated by using the original 48-bit long destination address, or obtained by using another method. We call the short destination address as PBSSID (Partial BSSID), that is, a part of BSSID. However, no matter how, because the address is too short, an address conflict, that is, a PBSSID conflict, occurs easily. The situation when the conflict occurs is that, after a station (Station, STA for short) sends an NDP PS-Poll, two or more than two access points (Access Point, AP for short) receive the NDP PS-Poll at the same time, and those APs all consider, through determining, that the NDP PS-Poll is sent to themselves, because their PBSSIDs are the same. Because a downlink data frame is generally longer than an acknowledgement frame (Acknowledgement, ACK for short), in order to prevent that, in a case of a PBSSID conflict, interference occurs between a downlink data frame directly sent by an AP and a downlink data frame or an ACK acknowledgement frame sent by another AP at the same time, causing a waste of a relatively long time, one skilled in the art once proposed to a limitation that after receiving an NDP PS-Poll, an AP must send an ACK acknowledgement frame first, and then send downlink data, instead of sending the downlink data directly. However, in this case, the STA receives ACKs returned by a plurality of APs at the same time, and at this time, the ACKs have interference between each other, but the STA is incapable of distinguishing, and may only receive an ACK sent by an AP associated with the STA itself, and may also only receive an ACK sent by another certain AP. Similar to the PS-Poll, a new ACK structure is also proposed, and also has only a physical header. This type of new ACK frame structure is called short ACK. A short ACK carries an ACK_ID, where the ACK_ID carries some information. An STA that receives a short ACK identifies, by using the ACK_ID information, whether the short ACK is sent to the STA itself. An ACK_ID in the prior art is calculated by using some information in that frame acknowledged by the ACK_ID. For example, an STA sends a frame, e.g., frame1, to an AP, the AP obtains an ACK_ID through calculation by using some information of the frame1, and includes the ACK_ID in a short ACK, and sends the short ACK to the STA; after receiving the short ACK, the STA knows that the short ACK is sent to the STA itself, and is an acknowledgement reply to the frame1 sent by the STA itself.
However, during a research and practice procedure of this method, the inventor of the present invention finds that: When the frame1 is an NDP PS-Poll, if a PBSSID conflict occurs, a plurality of APs return short ACKs. However, ACK_IDs in the short ACKs are all calculated by referring to the same NDP PS-Poll. Therefore, the ACK_IDs are all the same. Then, after receiving the short ACKs, an STA is incapable of determining whether a short ACK is sent to the STA itself by an AP associated with the STA itself.